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If the main breaker in the main panel is 100A.

And you have a subpanel with only light load, say 20A to 30A each breaker totally at most 70A simultaneous use of all breakers.

Do you need to use wire gauge sized for 100A (AWG 2 instead of just AWG 4)? I'm aware you can use the 90 degree table.

Rational is if the subpanel feeder wire is small for the main panel main breaker, it would heat up? But what if the load won't reach that amperage. Then you can use smaller feeder wire just enough for the main breaker in main panel to trip if there is a short, and not deal with overloading which won't happen in the subpanel because the load is sized for only 70A and AWG 4?

Or does the rule say the subpanel feeder wire need to match the main panel main breaker no matter how light the load in subpanel so need to use AWG 2?

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  • You can't use the 90 degree table. 40A=#8 60A=#6
    – Harper - Reinstate Monica
    Jan 25, 2019 at 1:33
  • Why? The subpanel only has lugs and only 70 maximum load capacity?
    – Jtl
    Jan 25, 2019 at 1:50
  • @Harper, my subpanel is connected to the main panel directly via chair lug terminals in the main panel without using any smaller breaker but the main breaker itself. The main breaker is 125A. The subpanel lug & rating (without main breaker) is 125A. I used only 30mm^2 which is between AWG 3 & 2. In this table. cerrowire.com/products/resources/tables-calculators/… I can use AWG 2 for the 90 degree table (or 130A) since both are lug chair, right? Or if the main breaker in main panel is 75 degree, it means I should only use 75 degree? Thanks lot!
    – Jtl
    Jan 27, 2019 at 22:57
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    I am unclear. If the main breake is turned off, will this subpanel branch lose power? To start with, what is the type of wire?
    – Harper - Reinstate Monica
    Jan 27, 2019 at 23:02
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    That's a wire maximum, that doesn't mean you can use it. The 60C column generally, however there are other uprates allowed based on exactly what you are doing. If you shut off this main breaker, will the feed to the subpanel lose power?
    – Harper - Reinstate Monica
    Jan 27, 2019 at 23:08

2 Answers 2

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The feeder for the second panel, is sized to the load it's expected to serve. The breaker in the main panel, that protects the feeder to the second panel. Is sized to protect the feeder wire.

For example, if the load calculation shows that the second panel is expected to supply 60 amperes. Then the feeder to the panel, only has to be sized for the 60 ampere load. You'll then need a breaker in the main panel, that's sized to protect the feeder. So in the main panel, the feeder will have to be connected to a 60 ampere breaker.

tldr;

Feeders are sized to the load they'll serve. Breakers are sized to protect the wiring connected to them.

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  • But what if the main breaker of main panel is 100A?
    – Jtl
    Jan 25, 2019 at 1:49
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    @jtl you seem to be saying that an overload between 60A and 100A is too unlikely to bother protecting for. NFPA disagrees, and collecting and correlating fire reports is literally their business. They are the people who do that.
    – Harper - Reinstate Monica
    Jan 25, 2019 at 2:20
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    @Jtl I'm not sure what you're asking. The main breaker in the main panel shouldn't matter. There should be a breaker in the main panel, that specifically protects the feeder to the second panel. It's the same as feeding a branch circuit. You'll have 15 or 20 amp breakers protecting those.
    – Tester101
    Jan 25, 2019 at 4:14
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Expected load has nothing to do with it. The name of the game is protecting the wire between main and subpanel, and the subpanel itself, from a worst case. If we only planned for what "won't" happen, there'd be no need for breakers or grounds at all.

Inside the main panel will be a breaker which feeds the wiring to the subpanel. The lesser of those current ratings decides the size this breaker must be.

Example: you have a 200A subpanel connected via 6 AWG feeder. This must be fed by a 60A breaker in the main panel. The wire size is the limiting factor.

Example: you have a 125A rated subpanel fed by 0000 AWG feeder. This must be fed by a 125A or smaller breaker in the main panel. Panel rating is the limiting factor.

If the main panel's MAIN breaker is of a size that will adequately protect the feeder wires and the subpanel, then no breaker is needed.

Example: the main panel has a 100A main breaker. There is 00 AWG wire going to a 150A subpanel. The main panel's main breaker will adequately protect this feeder and subpanel, so it can be fed without a greaker.cc

You would either use a panel that has thru lugs, or a "lug breaker" (search term: sub-feed lug block) which fits where a breaker does, but is just lugs.

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Remember: subpanels NEVER need main breakers. A subpanel in an outbuilding needs a main shutoff switch. If you use a main shutoff switch that just happens to be a breaker also, whatever...

You can never tap the top (meter side) of the main breaker. The run to the subpanel must be protected.

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    So the bottom line is, the wires should match or can be bigger than the breaker rating to avoid the wires melting and catching fire before the breaker open? So if say the breaker in your main panel feeding the subpanel is 100A (let's say you don't have money to buy lower rating, or just for sake of discussion). And your load in the subpanel is only 5A (for exaggeration effect). You still need to use AWG 2 (rated for 100A) between it and the main panel, right?
    – Jtl
    Jan 25, 2019 at 3:30
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    @Jtl Yes. That is exactly the principle here. You got it. Double check your called out wire sizes, typically one uses 3 AWG, or smarter, 1 AWG Aluminum.
    – Harper - Reinstate Monica
    Jan 25, 2019 at 15:44
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    And yeah, if your main panel's main breaker is 100A, and your feeder wire is adequate size for 100A, then you do not need a second breaker protecting the feeder, the main has that effect. Mind you, a 60A breaker is ten bucks, it's the 70+ amp breakers that start to get expensive. That was the concept behind Rule of Six panels.
    – Harper - Reinstate Monica
    Jan 25, 2019 at 15:46

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